1. Field of the Invention
The present invention concerns transistor power amplifiers or SSPA (Solid-State Power Amplifiers) and in particular amplifiers using MESFET (MEtal-Semiconductor Field-Effect Transistor) power devices.
2. Description of the Prior Art
These amplifiers are used in space applications i.e. satellites for the final radio frequency amplification stages especially for microwave signals, for example the signals driving a travelling wave tube (TWT) designed to transmit a highpower communication signal. Such amplifiers are particularly advantageous in this field because of their high intrinsic reliability, their low mass, their compact dimensions and the ease with which they can be adapted to different applications. MESFET are virtually universally used to amplify signals at frequencies above 1.5 GHz.
The amplifiers operate in one of two modes, depending on how sensitive the signals to be amplified are to non-linearities of the amplifier which themselves generate distortion.
In the first or linear mode, which must be used to amplify signals which are particularly sensitive to such non-linearities such as multiple carrier signals, the operating point of the amplifier is adjusted so that the amplifier always operates below its rated power output so that there is a safety margin and very little distortion from the input to the output.
In the second mode, which can be used to amplify signals which are less sensitive to non-linearities such as QPSK (Quadrature Phase Shift Keying) signals on a single carrier, the safety margin referred to above is dispensed with which significantly increases the output power of the amplifier but however also increases distortion. This mode is sometimes called "saturation" mode.
In either case the operating temperature of the amplifier significantly effects its characteristics. This is particularly critical in space applications given the very wide range of temperature variation routinely encountered: the case of amplifiers on board satellites, for example, the temperature of the transistors typically varies between -10.degree. C. and +40.degree. C., with frequent and relatively sudden heating/cooling cycles.
An increase in temperature reduces drift mobility and the maximum drift speed of the electrons in the MESFET channel, so reducing the transconductance of the component and the maximum channel current. Thus as the temperature increases the gain and the output power of the amplifier are reduced. At low temperatures, on the other hand, the contrary effect on the gain and the output power leads to excessive power consumption by the amplifier.
Techniques for compensating these temperature effects have already been proposed.
To be more precise, in a first family of known techniques the gain of the amplifier is compensated globally by operating on a pre-amplifier stage on the input side of the power amplifier. The pre-amplifier stage includes a circuit whose gain varies as a function of temperature in the opposite direction to the variation in the gain of the other circuits of the amplifier so as to compensate the overall gain of the latter. The variable gain circuit may be a variable attenuator or a variable gain amplifier controlled by a component responsive to temperature or an automatic gain control circuit controlled by a circuit measuring the output power of the amplifier (by means of a coupler, for example) and adjusting the gain of the pre-amplifier stage to maintain a constant output level and therefore a constant overall gain of the amplifier system.
However it is implemented, this technique is effective in the field of linear operation but becomes ineffective at saturation because of the insufficient dependency of output power on input power: when the amplifier saturates, variation of the level at the output of the pre-amplifier stage varies the output power only slightly.
A method of this kind is thus unable to compensate correctly for the effect of temperature variations on an amplifier operating in saturation mode.
Another known compensation technique, rather than operating on the overall gain of the amplifier system, operates individually on the various MESFETs by using a current control circuit to increase the drain current of each component as the temperature rises: because (in the linear region) the gain is proportional to the transconductance of the component which in turn depends on the drain current the increase in the drain current compensates for the temperature-dependent reduction of the gain.
Although this technique is effective in theory, it has various drawbacks including:
the need to provide a control circuit for each component, which renders the amplifier more complex and therefore less reliable, heavier and larger, all these characteristics being particularly critical in space applications, PA1 the slope of the drain current/temperature curve is not constant, but varies with frequency; thus this technique cannot be applied to broadband amplifiers unless further compensation of frequency variation effects is provided, PA1 for the highest temperatures a strongly increased drain current may produce an excessive channel temperature, reducing the reliability of the MESFET, PA1 finally, as in the previous case, this technique is applicable only if transistor operation is essentially linear. PA1 at least one MESFET power circuit, and PA1 a power supply unit adapted to supply to said at least one power circuit the voltages necessary for the latter to operate and including a DC drain voltage, said power supply unit comprising: PA1 a temperature-sensitive component near the MESFET(s) of said at least one power circuit to sense their temperature, and PA1 means slaved to a parameter delivered by said temperature-sensitive component to vary said DC drain voltage in the same direction as the temperature varies to compensate by operation on said DC drain voltage the temperature-dependent antagonistic variation of gain and output power of said at least one power circuit.
It is thus clear that in each of these prior art techniques the gain of the amplifier is compensated, but only in the linear region. The output power variation is not compensated in the saturation region.
Unlike a bipolar transistor, the operating point of a MESFET cannot be controlled when the device is saturated by operating on the bias at the gate because control by the gate contact becomes less and less effective as saturation is approached. Any attempt to operate on the biasing of the saturated component entails the risk of damaging the component due to an excess gate current.
An object of the present invention is to alleviate these drawbacks and to this end to propose a new technique for compensating temperature effects in a MESFET amplifier which compensates for such effects completely, not only in the linear region but also--and especially--in the saturation region.
It will be shown that the effectiveness of the compensation technique in accordance with the invention is proportional to the degree of saturation, that it has only a very limited effect on reliability and that it does not cause any significant degradation of the efficiency of the power supply unit.
It will also be shown that the compensation technique in accordance with the invention is perfectly compatible with existing compensation techniques using a variable gain circuit. This possibility of simultaneous double compensation is particularly beneficial in applications where the same amplifier must operate in the linear region and in the saturated region, changeover from one operating mode to the other being remotely commanded. For example, in space applications some transmitters must be able to operate at very different output powers according to whether long-range or short-range transmission is required.